1. Field of the Invention
The present invention relates to an apparatus for desalting or softening water by electrodialysis; the apparatus includes a circular cylindrical tank in which are provided a central electrode which extends along the axis of the tank, a coaxial electrode chamber which embeds the central electrode in electrlyte solution, a coaxial resin chamber which contains ion exchange resin, and a coaxial brine chamber; the separation of adjoining chambers is effected by respective membranes, which are permeable to either cations or anons.
2. Description of the Prior Art
An apparatus of this general type embodies the combination of ion exchange material with a dialysis device and an electrolysis device. Characteristic of this combination is the continuous regeneration of the ion exchange material by the motive force of the electrical field and the separation action of the respectively adjoining membrane. In contrast, with a conventional ion exchanger, a regeneration phase and a wash phase follow a loading phase, considerable quantities of regeneration chemicals are required, considerable drainage or waste water losses result, the exchange or reversing process is generally provided with a second unit, and extensive control devices are required.
U.S. Pat. No. 3,686,089 Korngold et al dated Aug. 22, 1972 discloses an apparatus for desalting water by electrodialysis, according to which a tank is divided by membranes, which are either permeable to cations or to anions, into successive chambers for cation resin, anion resin, brine, an anode, and a cathode. Not only the anode chamber and the cathode chamber but also the cation resin chamber and the anion resin chamber are spaced from one another by the brine chamber, and the cation resin chamber and the anion resin chamber are connected in series by a connecting line for the partially desalted water. All of the membranes have a planar arrangement, have the same rectangular cut, and are disposed parallel to one another. The same also applies for the chambers for the cation resin, the anion resin, the brine, an anode, or a cathode, which chambers are partitioned by the membranes. A drawback of this heretofore known apparatus is that it requires four charging spaces, the maintenance of a given chamber frequently requires dismantling of the entire apparatus, sealing to a higher operating pressure causes difficulties, no uniform distribution of the liquid, which is to be treated and enters from above, is possible, the shunting of the gases which are formed is too irregular, increasing irregularities in the phase profile of a given ion exchange bed, which tend to escape of ions, cannot be corrected in a simple manner, and deposits which increase the resistance of the ion exchange bed to flow can be removed only at great expense.
An apparatus of the initially defined type is disclosed in co-pending German Patent Application No. 34 18 102.4, filed May 16, 1984 belonging to the assignee of the present invention and included as basis for the present disclosure. With this apparatus, the aforementioned tank comprises two circular cylindrical tanks, two or more flange junctions which extend radially relative to both of the tanks, and successively arranged in the radial direction in one of the tanks a circular cylindrical anode chamber, a hollow cylindrical cation resin chamber, and a hollow cylindrical brine partial chamber, and in the other tank a circular cylindrical cathode chamber, a hollow cylindrical anion resin chamber, and a hollow cylindrical brine partial chamber; the brine chamber is completed by the interior of the flange junctions. However, with this arrangement one cannot selectively utilize a given separate tank as a simple ion exchanger, i.e. as a cation exchanger or as an anion exchanger. This short coming in versatility complicates the applicability for apparatus for desalting water by electrodialysis, and precludes a more economical mass production. Furthermore, due to circulation in the brine chamber, it is difficult to obtain a greater homogeneity in the brine concentration, which is necessary for a greater efficiency of the apparatus. Moreover, in the region of the brine, limits are set for shortening the electrical field lines, and hence also for reducing the electrical rsistance and the operating costs of the apparatus.
An object of the present invention is to provide an apparatus for desalting water by electrodialysis, which aparatus is economical to manufacture and operate, has a versatile applicability, has a high efficiency when treating aqueous liquids, is convenient to transport, can operate with a small charging space, is not very susceptible to disturbance, and makes possible a simple maintenance.